Refactor @at-root to ensure that it works properly with a plain &.

This changes the structure of the intermediate trees a little. Instead
of having the RuleNode nesting resolved entirely in the Cssize
visitor, the selectors are now resolved in Perform, while the nesting
itself is removed in Cssize. This is necessary to make the resolved
selectors consistently available to script &.

Closes #957
Merge pull request #958 from nex3/at-root-refactor

Refactor @at-root to ensure that it works properly with a plain &.